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How bioswales manage stormwater

I love urban planning and finding ways we can incorporate nature into our cities. From green buildings, to vertical, urban forests, there are so many ways that we can imagine incorporating nature into our built environments.

One way I love seeing this done is through the addition of bioswales.

A bioswale filled with blue-green sedges, small bushes, and two small trees.

A bioswale in SE Portland

What are Bioswales?

Bioswales are "vegetated, shallow, landscaped depressions designed to capture, treat, and infiltrate stormwater runoff as it moves downstream." In Portland, these are also often filled with native plants, as they've evolved to handle our dry summers and wet winters.

We get bioswales filled with oregon grape, cascara trees, and all types of sedges. They're beautiful to look at year round, and add nature back into the built environment. To understand how impactful bioswales are, you first have to understand how stormwater works.

How does stormwater work?

When it rains, two separate things can happen. If rain falls on a heavily vegetated area, the water gets soaked up like a sponge. Some of the water may run off the top of the sponge, and some may slowly leak out, but overall the amount of water passing through is slowed down and soaked into the earth, contributing to groundwater.

When water lands on impervious surfaces (think roads and sidewalks) it becomes surface runoff. There's nothing for it to soak into so it either pools or flows downstream.

As it flows, it picks up debris. In cities, that means it picks up a lot of pollutants like fertlizer, oil, pesticides, and microplastics from tires.

A diagram showing how rain water flows in a storm. The main caption reads "Stormwater: Where it flows, everything goes. When it rains, snows, or sleets, water hits hard surfaces and takes anything on that surface with it, through drains, pipes, and ditches to local rivers, lakes, and streams.

EPA stormwater diagram

How do bioswales help with stormwater?

In Portland, that surface runoff eventually ends up in our river. Because bioswales are filled with vegetation, they help a couple different stormwater issues.

First is erosion. When water meets little resistance, it flows quickly. If the ground beneath it is just dirt or pavement, the water isn't soaked up and either erodes away the dirt or pools and flows off the pavement. In extreme cases this results in landslides or flooding.

Because bioswales have vegetation, soil, and an aggregate (usually sand), the water pours in and is slowed down by the soil, helping to prevent flooding.

Next is infiltration. As the water seeps into the soil, some of the water is taken up by the roots of plants. The remainder goes deeper into the ground, settling into the soil. This contributes to ground water.

An illustration that shows how bioswales work. Rainwater hits the ground and flows into the bioswale. The water is soaked up by the soil and roots of plants in the bioswale.

Bioswale illustration by Kristy Beyer

Adding bioswales are an innovative way to add nature back into our landscape while improving a town's stormwater management and safety. Plus, bioswales save money in building costs. It's really great seeing more of these being built throughout the city!

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